Field of the Invention
The present invention relates to thermocouples. More specifically, the present is directed to a cold junction compensation apparatus for a thermocouple.
Devices for providing cold junction referencing of thermocouples are well-known in the art as shown in U.S. Pat. Nos. 4,157,663 and 4,147,061. Such cold junction compensation is necessary since thermocouples will only provide accurate results in measuring the temperature at the so-called hot junction when the cold junction is at a known temperature. As a result, the output signal from the thermocouple is thereby influenced only by temperature changes at the hot junction rather than the net effect of temperature changes at both the hot and cold junctions. Prior attempts to provide such cold junction compensation have included maintaining the temperature of the cold junction by a thermostatically controlled means in order to provide a constant temperature environment for the cold junction. While that system provides a constant temperature environment for the cold junction, it limits the range of temperature which may be measured by the thermocouple due to the limiting temperature of the cold junction and also requires the use of thermostatic controls for maintaining a constant temperature at the cold junction. Another prior art form of cold junction compensation has included the use of a compensating resistor in a temperature measuring means to compensate for variations in the temperature of the cold junction. Such a solution requires that the value of the resistor be changed whenever different thermocouple materials are used or when the instrument is relocated in a different temperature environment. Another prior art temperature conditioning system is shown in U.S. Pat. No. 4,147,061 which uses a complicated and expensive reference block approach for providing isothermal conditioning of the thermocouple reference junction. This preconditions the lead wire to the same temperature as the reference block and prevents the lead wire from absorbing heat from the reference junction to minimize temperature induced errors. All input leads in that apparatus are connected by electrical screws to terminals on one side of the reference block and output leads are attached to terminals on the opposite side of the block. A calibrated temperature monitoring device is attached to the reference block, and the entire block along with temperature insulating material is enclosed in a case. A multiplicity of connector terminals are arranged to abut an aluminum plate which presses against an insulating rear face of each connector. The aluminum plate acts as an isothermal heat sink for all of the connectors while a temperature sensitive resistor immediately adjacent to the center of the aluminum plate makes it possible to measure and calibrate the temperature of all the reference junctions. Such prior art attempts to provide a cold junction compensation apparatus have various respective limitations which prevent them from being widely adopted in a cost-effective manner. Accordingly, it would be desirable to provide a cold junction compensation apparatus which avoids the complexity and cost of the prior art devices.